Capsule-shaped polymer particles and process for the production thereof

ABSTRACT

Capsule-shaped polymer particles and a process for the production thereof are described. In one embodiment of the process, a polymerizable monomer component containing a cross-linkable monomer and a hydrophilic monomer, and an oily substance are finely dispersed in water to prepare an oil-in-water emulsion and then the polymerizable monomer component is polymerized, whereupon capsule-shaped polymer particles containing the oily substance as a core material are obtained. Upon removal of the oily substance in the particles, there are obtained capsule-shaped hollow polymer particles. These capsule-shaped polymer particle are excellent in such properties as mechanical strength and heat resistance, and thus can find many applications.

TECHNICAL FIELD

The present invention relates to capsule-shaped polymer particles formedby utilizing phase-separation between a polymer and an oily substance inthe inside of particles during polymerization, hollow capsule-shapedpolymer particles formed from such capsule-shaped polymer particles, anda process for the production thereof.

BACKGROUND ART

For capsulation by covering a core in a fine particle form with apolymer, so-called microcapsulation, a number of methods are known,including the phase-separation method, the interfacial polymerizationmethod, the in situ polymerization method and the spray drying method.

In microcapsulation by the in situ polymerization method among the abovemethods, generally by carrying out polymerization in a dispersion systemin which a core material and reactants, i.e., reaction components suchas a monomer and a prepolymer necessary to form a polymer, or a catalystcoexist, a polymer insoluble in the core material and a dispersionmedium is formed in the interface therebetween and thus a shell of thepolymer is formed on the surface of particles of the core material,whereby capsulation is realized.

However, in microcapsulation by the conventional in situ polymerizationmethod, in practice, capsulation is rarely achieved as described above;it has disadvantages in that the polymer formed by polymerization doesnot sufficiently undergo phase-separation from the core particle and isentrained in the core particle, thereby failing to form a shell, or evenif the polymer is formed in the condition that it has undergonephase-separation from the core particle, the shell is formedinsufficiently and the core in the inside is exposed, and thuscapsulation is incomplete. Thus, in fact, the capsulation by the in situpolymerization method has not almost been put into industrial usealthough it has advantages in that the process is simple and the size ofthe capsule or the thickness of the shell is easy to control.

Heretofore, as methods for the production of hollow capsule-shapedpolymer particles (hereinafter referred to as "hollow polymerparticles"), for example, the following have been known:

(I) a method in which a foaming agent is incorporated in polymerparticles and afterwards the foaming agent is expanded;

(II) a method in which a volatile substance such as butane isincorporated in a polymer and afterwards it is gassified and expanded;

(III) a method in which a polymer is melted, and gas such as air isjetted into the molten polymer to thereby incorporate air bubbles in thepolymer; and

(IV) a method in which an alkali swelling substance is incorporated inthe inside of polymer particles, and an alkaline liquid is permeatedthrough the polymer particles to swell the alkali swelling substance.

However, these methods are all difficult in controlling conditions andit has been difficult to produce the desired hollow polymer particles inhigh yield and with high reliability.

DISCLOSURE OF INVENTION

The first object of the present invention is to provide capsule-shapedpolymer particles in the inside of which phase-separation between apolymer phase constituting a shell and an oily substance phaseconsituting a core are surely carried out and a hollow portion is surelyformed, and which have excellent characteristics in mechanical strength,heat resistance and so forth, and a process of production which permitseasy production of the above capsule-shaped polymer particles by asimplified process.

The above first object is attained by a process for the production ofcapsule-shaped polymer particles which is characterized by including astep where in an aqueous dispersion in which there is formed a dispersedphase in which a polymerizable monomer component containing across-linkable monomer and a hydrophilic monomer and an oily substancecoexist, the above polymerizable monomer is polymerized, andcapsule-shaped polymer particles produced by the above process ofproduction.

The above first object is more surely attained by a process ofproduction which is characterized in that in production ofcapsule-shaped polymer particles, in addition to a polymerizable monomercomponent and an oily substance, a polymer different from that resultingfrom polymerization of the above polymerizable monomer (hereinafterreferred to as a "different polymer") is introduced in a dispersed phasein an aqueous dispersion.

By coexisting a different polymer as well as the polymerizable monomerand the oily substance in a dispersed phase in an aqueous dispersion inthe production of capsule-shaped polymer particles, phase-separationbetween a polymer phase constituting the shell of the capsule-shapedpolymer and an oily substance constituting the core is surely carriedout and capsulation is attained more completely.

Furthermore, the above first object is more surely attained by a processof production which is characterized in that in the production ofcapsule-shaped polymer particles, in an aqueous dispersion where adifferent polymer is dispersed in the form of finely divided particles,a polymerizable monomer component and an oily substance are madeabsorbed on the finely divided particles of the different polymer andthereafter the polymerizable monomer is polymerized.

The second object of the present invention is to provide hollow polymerparticles which are produced from capsule-shaped polymer particlesproduced by the above process of production, and a process of productionwhich permits sure production of such hollow polymer particles by asimplified process.

The above second object is attained by a process for the production ofhollow polymer particles which is characterized by including a step ofremoving the oily substance in the capsule-shaped polymer particlesproduced by the above process of production, and hollow polymerparticles produced by this process of production.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 to 4 are micrographs showing the particle condition ofcapsule-shaped polymer particles or hollow polymer particles produced inexamples of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The polymerizable monomer component as used herein comprises (a) across-linkable monomer, (b) a hydrophilic monomer, and (c) other monomerwhich is used if desired.

As the cross-linkable monomer (a), divinyl monomers such asdivinylbenzene, ethylene glycol dimethacrylate, 1,3-butylene glycoldimethacrylate, trimethylolpropane trimethacrylate, and allylmethacrylate, and trivinyl monomers can be given. Of these monomers,divinylbenzene, ethylene glycol dimethacrylate, and trimethylolpropanetrimethacrylate are particularly preferred.

The amount of the cross-linkable monomer used is usually 1 to 50 partsby weight, preferably 2 to 20 parts by weight per 100 parts by weight ofthe total weight of the polymerizable monomer component. The amount ofthe cross-linkable monomer is calculated as a pure monomer excluding aninert solvent and a monocyclic non-cross-linkable monomer componentusually contained in a cross-linkable monomer material. If the amount ofthe cross-linkable monomer used is too small, phase-separation betweenthe polymer constituting the shell and the oily substance constitutingthe core is accomplished insufficiently, or a problem arises in that thestrength of the shell is poor, so that the final capsule-shaped polymerparticles cannot maintain its shape.

On the other hand, if the amount of the cross-linkable monomer used istoo large, the polymer different from that resulting from polymerizationof the polymerizable monomer component tends to be expelled to theoutside of polymer particles formed during polymerization, as a resultof which a problem arises in that the final polymer particles are not ina true spherical form and only block-shaped particles havingirregularities in the surface thereof are obtained.

As the hydrophilic monomer (b), vinyl monomers such as vinylpyridine,glycidyl acrylate, glycidyl methacrylate, methyl acrylate, methylmethacrylate, acrylonitrile, acrylamide, methacrylamide,N-methylolacrylamide, N-methylolmethacrylamide, acrylic acid,methacrylic acid, itaconic acid, fumaric acid, vinyl acetate,dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate,2-hydroxyethyl methacrylate, and 2-hydroxypropyl methacrylate can begiven. Particularly preferred are methyl methacrylate, vinylpyridine and2-hydroxyethyl methacrylate, and not more than 20 wt % of unsaturatedcarboxylic acids such as methacrylic acid.

The solubility in water of the hydrophilic monomer is preferably notless than 0.5 wt % and particularly preferably not less than 1 wt %.Although the optimum amount of the hydrophilic monomer used varies withthe type of the oily substance constituting the core, it is usually 3 to99 parts by weight, preferably 5 to 99 parts by weight, and particularlypreferably 50 to 95 parts by weight per 100 parts by weight of the totalweight of the polymerizable monomer component.

When unsaturated carboxylic acid is used as the hydrophilic monomer, theamount of the unsaturated carboxylic acid used is preferably not morethan 20 wt %. When the hydrophilic monomer is vinylpyridine or2-hydroxyethyl methacrylate, the amount of the hydrophilic monomer usedis preferably not more than 40 wt %.

If the amount of the hydrophilic monomer used is too small, a problemarises in that phase-separation between the polymer constituting theshell and the oily substance constituting the core does not occur oroccurs only insufficiently and thus the polymer fails to completelycover the whole core particle, leading to unsatisfactory capsulation.

As the monomer (c) which is used if desired, any monomers can be used aslong as they are radical polymerizable. Typical examples are aromaticvinyl monomers such as styrene, alpha-methylstyrene, p-methylstyrene,and halogenated styrene, vinyl esters such as vinyl propionate,ethylenically unsaturated carboxylic acid alkyl esters such as ethylmethacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate,2-ethylhexyl methacrylate, lauryl acrylate, and lauryl methacrylate, andconjugated diolefins such as butadiene and isoprene. Of these monomers,styrene is particularly preferred.

In the present invention, in order to more accelerate phase-separationin the inside of polymer particles during polymerization, it isparticularly preferred that a polymer different from that resulting frompolymerization of the polymerizable monomer component be previouslyadded to an aqueous dispersion medium. It is necessary for thisdifferent polymer to be a polymer of type or composition different fromthat resulting from polymerization of the above polymerizable monomers(a) to (c), and further to be soluble in the polymerizable monomer oroily substance.

Typical examples of the polymer different from that of the polymerizablemonomer component are polystyrene, carboxyl-modified polystyrene, astyrene-butadiene copolymer, a styrene-acrylate copolymer, astyrenemethacrylate copolymer, an acrylate copolymer, a methacrylatecopolymer, a carboxyl-modified styrene-acrylate copolymer, acarboxyl-modified styrene-methacrylate copolymer, a carboxyl-modifiedacrylate copolymer, and a carboxyl-modified methacrylate copolymer.

Of these polymers, polystyrene or a styrene copolymer containing atleast 50 wt % of a styrene component is preferred.

The amount of the polymer as defined above being used is 1 to 100 partsby weight, preferably 2 to 50 parts by weight, and more preferably 5 to20 parts by weight per 100 parts by weight of the total weight of thepolymerizable monomer component. If the amount of the polymer used isless than 1 part by weight, the effect of acceleration ofphase-separation is poor. On the other hand, if the amount of thepolymer used is in excess of 100 parts by weight, a problem arises inthat phase-separation rather tends to be prevented.

In polymerizing the polymerizable monomer component in the presence ofthe polymer different from that resulting from polymerization of thepolymerizable monomer component, the following methods can be employed.(1) The different polymer is used in the form of solid particles. Thesesolid particles are dispersed in an aqueous medium and after thepolymerizable monomer component and the oily substance are added to theabove dispersion and absorbed in the finely divided polymer particles,the polymerizable monomer component is polymerized; (2) The polymer isdissolved in the polymerizable monomer component and/or the oilysubstance to prepare a solution, and after the solution is dispersed inwater, the polymerizable monomer component is polymerized.

When the different polymer is used in a particle state by the method(1), the finely divided polymer particles act as seed polymer particlesand the polymerizable monomer component and the oily substance areabsorbed therein. Thus it is preferred for the polymer to have a highcapacity to absorb the polymerizable monomer component and the oilysubstance. For this reason, it is preferred for the polymer to have alow molecular weight. For example, the number average molecular weightis not more than 20,000, preferably not more than 10,000, and morepreferably not more than 7,000. The number average molecular weight asused herein is determined by usual procedures such as by dissolving thepolymer in its good solvent and measuring the resulting solution bytechniques such as gel permeation chromatography (GPC) or by the use ofa molecular weight-measuring device by means of osmotic pressure orvapor pressure depression.

If the number average molecular weight of the polymer is more than20,000, the amount of monomers not absorbed in the seed polymerparticles is increased, and these monomers remaining unabsorbed arepolymerized independently from the seed polymer particles, resulting information of a large amount of fine polymer particles not formingcapsules and causing a problem that the polymerization system becomesunstable.

The polymer used as such seed polymer particles is not critical in itsprocess of preparation. For example, the polymer can be produced bytechniques such as emulsion or suspension polymerization using arelatively large amount of a chain transfer agent.

In a case that the polymer different from that resulting frompolymerization of the polymerizable monomer component is used as seedpolymer particles, the capacity of the polymer particles to absorb thepolymerizable monomer component and the oily substance can be increasedby making the polymer particles absorb a highly lipophilic substancehaving a solubility in water of not more than 10⁻³ wt % prior toaddition of the polymerizable monomer component and the oily substance.

When there is employed a means of making the highly lipophilic substanceabsorbed on the seed polymer particles, the number average molecularweight of the polymer may be more than 20,000.

Typical examples of the highly lipophilic substance are1-chlorododecane, octanoyl peroxide, and 3,5,5-trimethylhexanoylperoxide.

In making the highly lipophilic substance absorbed in the seed polymerparticles, it suffices that an aqueous emulsion of the highly lipophilicsubstance is prepared and mixed with an aqueous emulsion of the seedpolymer particles to thereby bring the substance into contact with theparticles.

Capsule-shaped polymer particles and hollow polymer particles producedusing seed polymer particles have a particle diameter which is nearlyequal to that of the seed polymer particles which have swollen as aresult of absorption of the polymerizable monomer and the oilysubstance. Thus the particle diameter of capsule-shaped polymerparticles and hollow polymer particles being formed can be controlled byappropriately controlling the particle diameter of seed polymerparticles, a ratio of the amount of the seed polymer particles to thatof the polymerizable monomer component and the oily substance, and soforth.

Specifically, in the production of hollow polymer particles, to produceparticles having a particle diameter of 0.3 to 0.6 μm, which areexcellent in whiteness and hiding power, it is sufficient to use seedpolymer particles having a particle diameter of 0.1 to 0.4 μm.

In producing capsule-shaped polymer particles or hollow polymerparticles having a particle diameter as small as 1 μm or less, it ispreferred to use seed polymer particles in that monomer droplets havinga small particle diameter can be easily and stably formed.

When the polymer different from that resulting from polymerization ofthe polymerizable monomer component is used in the method (2) asdescribed above, the molecular weight of the polymer is not critical.Preferably the number average molecular weight of the polymer is notless than 20,000.

The oily substance as used herein is not critical as long as it is ofsuch lipophilic property that the solubility in water is not more than0.2 wt %. Any of vegetable oil, animal oil, mineral oil and syntheticoil can be used. Typical examples of the oily substance are lard oil,olive oil, coconut oil, castor oil, cotton seed oil, kerocene, benzene,toluene, xylene, butane, pentane, hexane, cyclohexane, carbon disulfide,and carbon tetrachloride.

In addition, high boiling oils such as eugenol, geraniol, cyclamenaldehyde, citronellal, dioctyl phthalate, and dibutyl phthalate can beused. When these high boiling oils are used, capsule-shaped polymerparticles containing perfume, platicizers and the like in the insidethereof result.

The amount of the oily substance used is usually 1 to 1,000 parts byweight per 100 parts by weight of the total weight of the polymerizablemonomer component, with the range of 5 to 300 parts by weight beingpreferred. Inert solvents ordinarily contained in a commerciallyavailable cross-linkable monomer material can be deemed as oilysubstances, and thus the amount of the inert solvent is added as that ofthe oily substance.

If the amount of the oily substance used is too small, the amount of thecore is decreased and thus capsulation cannot be achieved. On the otherhand, if the amount of the oily substance used is too large, the amountof the monomer component becomes insufficient, resulting in a reductionin the film thickness of the outer shell of the polymer formed. Thus thefinal capsule-shaped polymer particles are poor in strength and arereadily collapsed.

The term "oily substance" as used herein includes, in addition to theabove-described oily substances, polymerizable monomers as describedabove. In this case, during the polymerization process, polymerizationis terminated in the condition that the polymerizable monomer remainsunpolymerized in the inside of the polymer particles formed, and thisresidual monomer is used as a substitute for the oily substance. In thiscase, it is necessary that the polymerization yield be adjusted to notmore than 97%, preferably not more than 95%. This can be attained byemploying techniques such as a method in which a small amount of apolymerization inhibitor is added, a method in which the temperature ofa reaction system is lowered at an intermediate stage of polymerization,and a method in which a polymerization terminator is added at anintermediate stage of polymerization.

Various substances which need physical protection against degradation,evaporation, pressure during handling, and so forth by means ofcapsulation, such as dyes, detergents, ink perfume, adhesives,medicines, agricultural chemicals, fertilizer, fats and oils,foodstuffs, enzymes, liquid crystals, paints, rust preventives,recording materials, catalysts, chemical reactants, magnetic substances,and so on can be dissolved or dispersed in the oily substance as usedherein depending on the purpose of use.

It is believed that polymerization in the process of the presentinvention is substantially suspension polymerization and s carried outin finely dispersed oil droplets containing the polymerizable monomercomponent, the oily substance, and preferably, the fine particles of thepolymer different from that resulting from polymerization of thepolymerizable monomer component, and that a polymer phase resulting fromphase-separation is formed on the outside of an oily substance phase,thereby forming capsule-shaped polymer particles.

In polymerization of the present invention, surface active agents orsuspension protective agents, organic or inorganic, which are commonlyused in usual polymerization, can be used as dispersion stabilizers.

In general, when capsule-shaped polymer particles having a particlediameter as small as less than about 1 μm are intended to produce, thesurface active agent is mainly used. On the other hand, whencapsule-shaped polymer particles having a particle diameter of more thanabout 1 μm are intended to produce, the suspension protective agent ismainly used.

Typical examples of the surface active agent are anionic surface activeagents such as sodium dodecylbenzenesulfonate, sodium laurylsulfate,sodium dialkylsulfosuccinate, and a naphthalenesulfonate-formalincondensate. Nonionic surface active agents such as polyoxyethylenenonylphenyl ether, polyethylene glycol monostearate, and sorbitanmonostearate can be used in combination.

Typical examples of the organic suspension protective agent arehydrophilic synthetic polymeric substances such as polyvinyl alcohol,polyvinyl pyrrolidone, and polyethylene glycol, natural hydrophilicpolymeric substances such as gelatine and water-soluble starch, andhydrophilic semi-synthetic polymeric substances such as carboxymethylcellulose.

Typical examples of the inorganic suspension protective agent are thephosphates of magnesium, barium, potassium and the like, calciumcarbonate, magnesium carbonate, zinc white, aluminum oxide, and aluminumhydroxide.

In the process of the present invention, as a polymerization initiator,any of oil-soluble and water-soluble polymerization initiators can beused.

However, in a case that the polymer different from that resulting frompolymerization of the polymerizable monomer component is used in theform of seed polymer particles and capsule-shaped polymer particleshaving a particle diameter as small as less than about 1 μm are intendedto produce, it is preferred to use a water-soluble polymerizationinitiator. Use of such water-soluble polymerization initiators enablesto prevent polymerization in large-sized monomer droplets not absorbedin the seed polymer particles.

In other cases, it is preferred that the oil-soluble polymerizationinitiator be used to prevent the formation of unnecessary new polymerparticles in addition to the desired capsule-shaped polymer particles.

Typical examples of the water-soluble polymerization initiator arepersulfates, and redox initiators such as hydrogen peroxide-ferrouschloride, and cumene hydroperoxide-sodium ascorbinate.

Typical examples of the oil-soluble polymerization initiator are benzoylperoxide, lauroyl peroxide, tert-butylperoxy-2-ethylhexanoate, andazobisisobutylonitrile.

In the process of the present invention, in many cases, a rate ofpolymerization is high because the amount of the cross-linkable monomerused is large. For this reason, in a case that polymerization is carriedout in a large-sized polymerization reactor, if all the polymerizationcomponents are introduced in the polymerization reactor and polymerizedat the same time, the temperature of the polymerization system becomesdifficult to control and there is a danger of run-away of thepolymerization reaction. In the process of the present invention,therefore, in order to avoid the above danger, it is preferred to employthe so-called increment polymerization method in which the monomercomponent is introduced, continuously or batchwise, in a polymerizationreactor during the polymerization process as such or in the form of anemulsion.

It is preferred that the polymerizable monomer component, the oilysubstance, and the finely divided polymer particles be introduced in thepolymerization reactor in such an emulsion state that they coexist inthe same particle or droplet.

In a case that the oily substance constituting the core of thecapsule-shaped polymer particles is a solvent or monomer of relativelyhigh volatility, such as benzene, toluene, xylene, butane, pentane,hexane, cyclohexane, and methyl methacrylate, the oily substance can beeasily replaced with water by applying pressure reduction treatment,steam stripping treatment and gas bubbling treatment, alone or incombination with each other, onto a dispersion of the capsule-shapedpolymer particles and, as a result, hollow polymer particles containingwater in the inside thereof can be obtained.

When the capsule-shaped polymer particles containing the oily substanceor those hollow particles containing water as prepared by replacement ofthe oily substance with water using the above-described treatment areseparated from water and dried, there can be obtained polymer particlesthe inside of which is empty, i.e., hollow polymer particles. With thesehollow polymer particles, the inner diameter is usually 1/4 to 3/4 ofthe outer diameter.

The above hollow polymer particles are useful as plastic pigments whichare excellent in such properties as luster and hiding power. Thecapsule-shaped polymer particles containing the oily substance in theinside thereof, or hollow polymer particles produced by introducing auseful component such as perfume, medicines, agricultural chemicalswhich are produced by the present invention, and an ink component in theinside of hollow polymer particles by techniques such as dipping, ordipping under pressure or reduced pressure can be used in various fieldsdepending on the useful component contained thereof.

EXAMPLES

The present invention is described below in greater detail although itis not intended to be limited thereto.

EXAMPLE 1

    ______________________________________                                        Monomer Component                                                             4-vinylpyridine      50         g                                             Divinylbenzene       2          g                                             Styrene              28         g                                             n-butyl acrylate     20         g                                             Oily Substance                                                                Toluene              100        g                                             Polymerization Initiator                                                      3,5,5-trimethylhexanoyl peroxide                                                                   2          g                                             (Perroyl 355 produced by                                                      Nippon Oil & Fats Co., Ltd.)                                                  ______________________________________                                    

The above ingredients were mixed to prepare an oily solution. Thissolution was added to 400 g of water in which 10 g of polyvinyl alcohol(Gosenol GH 20 produced by Nippon Gosei Kagaku Co., Ltd.) had beendissolved, and the resulting mixture was stirred for 15 minutes at 5,000r.p.m. by the use of a propeller-type stirring blade having a diameterof 5 cm to disperse the oily solution in water in such a manner that thediameter of oil droplets was 5 to 15 μm. The emulsion thus prepared wasplaced in a 1,000-milliliter separable flask. While stirring theemulsion in a nitrogen atmosphere at 120 r.p.m., polymerization wasperformed at a temperature of 70° C. for 15 hours to produce polymerparticles. The polymer yield was 98%.

Optical microscopic examination confirmed that the polymer particleswere spherical microcapsules comprising a core and a polymer shellcovering the whole surface of the core, and having an average particlediameter of 10 μm. A microphotograph of these polymer particles is shownin FIG. 1.

The flask was opened, and the dispersion of the capsule-shaped polymerparticles was allowed to stand for several hours. Then the polymerparticles were taken out of the flask. Optical microscopic examinationconfirmed that the polymer particles were hollow particles the outerwalls of which were partially depressed. This microphotograph is shownin FIG. 2.

COMPARATIVE EXAMPLE 1

Polymer particles were produced in the same manner as in Example 1except that 4-vinylpyridine (50 g) was not used and the amount ofstyrene used was changed from 28 g to 78 g.

Optical microscopic examination showed that no phase-separation occurredin the inside of the particles and the particles were uniform; that is,capsulation did not occur.

COMPARATIVE EXAMPLE 2

Polymer particles were produced in the same manner as in Example 1except that divinylbenzene (2 g) was not used and the amount of styreneused was changed from 28 g to 30 g.

Optical microscopic examination showed that no phase-separation occurredin the inside of the particles and the particles were uniform; that is,capsulation did not occur.

EXAMPLE 2

Two hundred grams of octanoyl peroxide (Perroyl O produced by Nippon Oil& Fats Co., Ltd.) was added to 2,000 g of water containing 15 g ofsodium laurylsulfate and then finely dispersed therein by the use of aManton-Gaulin homogenizer "Model 15M" produced by Manton-Gaulin Co.,Ltd. in such a manner that the particle diameter was 0.1 to 0.4 μm. Tothe emulsion thus prepared, 100 g of polystyrene particles (averageparticle diameter: 0.50 μm; average molecular weight: 105,000) as seedpolymer particles and 900 g of water were added, and further 800 g ofacetone was added. The resulting mixture was slowly stirred at atemperature of 10° C. for 20 hours to thereby make octanoyl peroxideabsorbed in the polystyrene particles. Then the acetone was removedunder reduced pressure to obtain a seed particle dispersion.

In a 1-liter separable flask was placed 32.2 g of the above seedparticle dispersion. In addition, 400 g of water, 1.35 g of sodiumlaurylsulfate, and 1 g of polyoxyethylene nonylphenyl ether (Emulgen 931produced by Kao-Atlas Co., Ltd.), and the following mixture were added.

    ______________________________________                                        2-hydroxyethyl methacrylate                                                                       20         g                                              Divinylbenzene      20         g                                              n-butyl acrylate    10         g                                              Styrene             50         g                                              Benzene             100        g                                              ______________________________________                                    

The resulting mixture was stirred at a temperature of 40° C. for 2 hoursto make the above substances absorbed in the seed polymer particles. Thetemperature of the system was raised to 70° C., and polymerization wasperformed for 10 hours to produce polymer particles.

Optical microscopic examination confirmed that the polymer particleswere spherical microcapsules comprising a core and a polymer shellcovering the whole surface of the core. With these polymer particles,the particle diameter was uniform, the average particle diameter was 2.7μm, and the standard deviation value of particle diameters was not morethan 0.1 μm. This microphotograph is shown in FIG. 3.

On drying the polymer particles in a vacuum drier, the benzene containedin the polymer was removed, and there were obtained hollow polymerparticles.

EXAMPLE 3

A latex oontaining styrene-butadiene copolymer particles (butadienecontent: 45%) having a particle diameter of 0.25 μm and a number averagemolecular weight of 6,800 was produced by emulsion polymerization usinga large amount of tert-dodecylmercaptan as a molecular weight modifier.

This latex was used as seed polymer particles. Four parts (2 parts assolids) of the latex, 30 parts of a 1% aqueous solution of sodiumlaurylsulfate, 20 parts of a 5% aqueous solution of polyvinyl alcohol,and 100 parts of water were uniformly mixed.

A mixture of the following substances:

    ______________________________________                                        Methyl methacrylate                                                                              80         parts                                           Divinylbenzene     20         parts                                           Toluene            30         parts                                           Benzoyl peroxide   2          parts                                           0.1% Aqueous solution of                                                                         400        parts                                           sodium laurylsulfate                                                          ______________________________________                                    

was finely dispersed by the use of a supersonic dispersing machine andthen added to the above-prepared uniform mixture. On slowly stirring theresulting mixture for 3 hours, the above monomer component and the oilysubstance were uniformly absorbed in seed polymer particles of thestyrene-butadiene copolymer.

Upon polymerization of the mixture at 75° C. for 6 hours, polymerparticles were obtained in a polymer yield of 98%.

This polymer particle dispersion was coated on a glass plate and wasallowed to stand at ordinary temperature for 10 minutes. Then the waterand toluene were evaporated and polymer particles were obtained. Opticalmicroscopic examination confirmed that the polymer particles were hollowpolymer particles having a high whiteness.

These hollow polymer particles were examined with a transmission-typeelectron microscope. This examination showed that the particles werespherical and hollow particles with no depression, having are outerdiameter of 0.95 μm and an inner diameter of 0.5 μm.

EXAMPLE 4

    ______________________________________                                        Divinylbenzene   10          g                                                Styrene          75          g                                                Benzoyl peroxide 5           g                                                ______________________________________                                    

The above ingredients were mixed to prepare a solution. To this solutionwas added 100 g of a magnetic fluid (Marpo Magna produced by MatsumotoOil & Fats Co., Ltd.), and they were uniformly mixed. The mixture thusprepared was added to 800 g of water in which 10 g of polyvinyl alcohol(Gosenol GH 20 produced by Nippon Gosei Kagaku Co., Ltd.) had beendissolved, and then stirred at 4,000 r.p.m. by the use of a T.K.autohomomixer (produced by Tokushukika Kogyo Co., Ltd.) to therebydisperse the above mixture in water in such a manner that the averageparticle diameter of oil droplets was about 10 μm. Then, 15 g of4-vinylpyridine was further added, and the resulting mixture wasstirred. The emulsion thus prepared was placed in a 2-liter separableflask, and polymerization was performed in a nitrogen atmosphere at atemperature of 80° C. for 8 hours to produce polymer particles.

Optical microscopic examination confirmed that the polymer particleswere spherical microcapsules comprising a core of the magnetic fluid anda shell of the polymer covering the whole surface of the core.

COMPARATIVE EXAMPLE 3

Polymer particles were produced in the same manner as in Example 4except that 4-vinylpyridine (15 g) was not used and the amount ofstyrene used was changed from 75 g to 90 g.

Optical microscopic examination showed that although phase-separationlocally partially occurred in the inside of the particles, capsulationwas accomplished insufficiently, and part of the core material wasexposed.

EXAMPLE 5

Ten parts of a commercially available polystyrene (produced byShin-Nittetu Kagaku Co., Ltd.; number average molecular weight: 150,000)was dissolved in a mixture of 20 parts of toluene, 90 parts of methylmethacrylate, and 10 parts (calculated as pure divinylbenzene) ofdivinylbenzene. The solution thus prepared was added to 400 parts ofwater with 10 parts of polyvinyl alcohol dissolved therein, andpolymerization was performed at 80° C. for 4 hours while stirring. Anaqueous dispersion of polymer particles having a particle diameter of 3to 8 μm was obtained in a polymer yield of 98%. Optical microscopicexamination confirmed that the polymer particles were capsule-shapedparticles having a double profile.

This aqueous dispersion of polymer particles was subjected to steamstripping treatment by blowing steam therethrough. The toluene in theinside of the polymer particles was removed, and hollow polymerparticles containing water in the inside thereof was obtained.

The water-containing polymer particles and the toluene-containingpolymer particles were placed on a slide glass and examined withoutplacing a cover glass. It was observed that both the water and toluenevaporized in 1 to 2 minutes and the particles changed to hollowparticles. These hollow particles were all completely spherical hollowparticles with no depression, and the ratio of outer diameter to voiddiameter (hereinafter referred to as a "outer diameter/inner diameterratio") was about 10:6.

EXAMPLES 6 TO 13, AND COMPARATIVE EXAMPLES 4 TO 6

Polymer particles were produced in the same manner as in Example 5except that the amount of the polystyrene resin, the composition of thepolymerizable monomer component, and the type and amount of the oilysubstance were changed in Table 1. They are called Examples 6-13,Comparative Examples 4-6.

These polymer particles were measured for the shape in the dispersionafter polymerization, the shape after drying, and the ratio of outerdiameter to void diameter (outer diameter/inner diameter ratio) in thesame manner as in Example 5. The results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                            After Drying                                                                       Inner                                                              Particle   Diameter/                        Polystyren                                                                              Monomer       Oily Substance                                                                          Shape      Outer                            Run Resin Composition        Amount                                                                             after Poly-                                                                         Particle                                                                           Diameter                         No. (parts)                                                                             (parts)       Type (parts)                                                                            merization                                                                          Shape                                                                              (μm/μm)                    __________________________________________________________________________    Ex. 5                                                                             10    MMA/DVB = 90/10                                                                             toluene                                                                            20   A     B    1.7                              Ex. 6                                                                             10    MMA/DVB = 55/45                                                                             toluene                                                                            20   A     B    1.7                              Ex. 7                                                                             10    MMA/ST/DVB = 60/39/1                                                                        toluene                                                                            20   A     B    1.8                              Ex. 8                                                                              2    MMA/ST/DVB = 60/30/10                                                                       toluene                                                                            20   A     B    1.8                              Ex. 9                                                                             70    MMA/ST/DVB = 60/30/10                                                                       toluene                                                                            100  A     B    1.3                              Ex. 10                                                                            10    MMA/DVB = 90/10                                                                             toluene                                                                            300  A     B    1.1                              Ex. 11                                                                            10    MMA/ST/DVB = 10/80/10                                                                       toluene                                                                            20   A     B    1.7                              Ex. 12                                                                            10    MA/ST/DVB = 3/87/10                                                                         toluene                                                                            20   A     B    1.7                              Ex. 13                                                                            10    MMA/DVB = 90/10                                                                             eugenol                                                                            30   A     A    1.6                              Com.                                                                              10    MMA/ST/DVB = 60/40/0                                                                        toluene                                                                            20   D     D    --                               Ex. 4                                                                         Com.                                                                              10    MMA/DVB = 40/60                                                                             toluene                                                                            20   E     E    --                               Ex. 5                                                                         Com.                                                                              10    MMA/ST/DVB = 0/90/10                                                                        toluene                                                                            20   F     F    --                               Ex. 6                                                                         __________________________________________________________________________     MMA: Methyl methacrylate                                                      DVB: Divinylbenzene                                                           ST: Styrene                                                                   MA: Methacrylic acid                                                          A: Oilcontaining capsuleshaped particle                                       B: Hollow particle                                                            D: Solid particle                                                             E: Cocoonshaped particle                                                      F: Porous particle                                                       

EXAMPLE 14

A mixture of 98 parts of styrene, 2 parts of methacrylic acid, and 10parts of tert-dodecylmercaptan was added to an aqueous solution preparedby dissolving 0.5 part of sodium laurylsulfate and 1.0 part of potassiumpersulfate in 200 parts of water, and polymerization was performed at70° C. for 8 hours while stirring to produce polymer particles. Thesepolymer particles had a particle diameter of 0.22 μm, atoluene-insoluble content of 3%, a number average molecular weight asdetermined by GPC of 4,100, and a weight average molecular weight(Mw)/number average molecular weight (Mn) ratio Mw/Mn=2.4.

These polymer particles were used as seed polymer particles. A mixtureof 10 parts as solids of the polymer particles, 0.1 part ofpolyoxyethylene nonylphenyl ether, 0.3 part of sodium laurylsulfate, and0.5 part of potassium persulfate was dissolved in 900 parts of water,and then a mixture of 80 parts of methyl methacrylate, 10 parts(calculated as pure divinylbenzene) of divinylbenzene, 10 parts ofstyrene, and 20 parts of toluene was added thereto. On stirring theresulting mixture at 30° C. for 1 hour, the above substances were almostcompletely absorbed in the seed polymer particles.

They were polymerized as such at 70° C. for 5 hours. A dispersion ofcapsule-shaped particles containing toluene in the inside thereof wasobtained at a polymer yield of 98%. After application of steam strippingtreatment onto the dispersion, the polymer particles were examined witha transmission-type electron microscope. It was found that the polymerparticles were seen through in the central part thereof and completelyspherical capsule-shaped particles. These capsule-shaped polymerparticles had an outer diameter of 0.51 μm and an inner diameter of 0.3μm.

An electron microscopic photography of the particles is shown in FIG. 4.

EXAMPLES 15 TO 18

Polymer particles were produced in the same manner as in Example 14except that as seed polymer particles, polymers having the monomercomposition, particle diameter, number average molecular weight, weightaverage molecular weight (Mw)/number average molecular weight (Mn)ratio, and toluene-insoluble content shown in Table 2 were used.

They are called Examples 15-18.

In Example 18, since the number average molecular weight of the polymerconstituting the seed polymer particles was as large as 23,000, thecapacity of the polymer to absorb the monomer component and the oilysubstance was small and thus, in addition to the desired capsule-shapedparticles, a large amount of fine particles having a particle diameterof about 0.05 μm, as formed by polymerization of oil droplets ofmonomers remaining unabsorbed was present and further the polymerizationsystem was instable.

EXAMPLES 19 AND 20, AND COMPARATIVE EXAMPLES 7 AND 8

Polymer particles were produced in the same manner as in Example 14except that as seed polymer particles, polymers having the monomercomposition, particle diameter, number average molecular weight, weightaverage molecular weight/number average molecular (Mw/Mn) ratio,toluene-insoluble content, and amount shown in Table 2 were used.

They are called Examples 19 and 20 and Comparative Examples 7 and 8.

In Example 20, the polymer particles were hollow particles having aslightly deformed shape.

In Comparative Example 7, since the amount of the seed polymer particlesused was decreased to 0.5 part, a number of monomers remained unabsorbedin the seed polymer particles. Thus a large amount of fine particlesformed by polymerization of the unabsorbed monomers are present and thesystem was geled.

In Comparative Example 8, since the amount of the seed polymer particlesused was as large as 150 parts, the amount of the seed polymer particleswas too large in relation to that of the polymerizable monomer andphase-separation did not proceed well in the inside of the particlesduring the polymerization process. Thus the particles were not hollowparticles but porous particles.

                                      TABLE 2                                     __________________________________________________________________________    Seed Polymer Particle                       Hollow                                                                   Amount                                                                             Particle                                                 Number     Toluene-                                                                           of   Outer                             Monomer           Particle                                                                           Average    insoluble                                                                          Seed Diameter/Inner                    Composition       Size Molecular  Content                                                                            Particle                                                                           Diameter Ratio                    (parts)           (μm)                                                                            Weight                                                                              Mw/Mn                                                                              (%)  (parts)                                                                            (μm/μm)                     __________________________________________________________________________    Ex. 15                                                                            ST/MA = 98/2  0.22 4,100 2.4   3   10   0.51/0.3                          Ex. 16                                                                            "             0.20 9.200 2.6   5   10   0.47/0.3                          Ex. 17                                                                            "             0.24 15,000                                                                              3.1  17   10   0.52/0.3                          Ex. 18                                                                            "             0.19 23,000                                                                              3.2  30   10   0.35/0.3                          Ex. 19                                                                            polystyrene   0.12 2,600 2.2   0   2    0.40/0.2                          com.                                                                              "             0.12 2,600 2.2   0   0.5  gelation                          Ex. 7                                                                         Ex. 20                                                                            ST/MMA/BD = 60/30/10                                                                        0.35 7,200 3.2  22   50   0.52/0.3                          com.                                                                              "             0.35 7,200 3.2  22   150  0.45 μm                        Ex. 8                                       (porous                                                                       particles)                        __________________________________________________________________________     ST: Styrene                                                                   MA: Methacrylic acid                                                          BD: Butadiene                                                                 MMA: Methyl methacrylate                                                 

EXAMPLES 21 TO 31, AND COMPARATIVE EXAMPLES 9 TO 11

Polymer particles were produced in the same manner as in Example 14except that as seed polymer particles, 10 parts of the same particles asused in Example 14 were used, and as the monomer and the oily substance,those shown in Table 3 were used.

They are called Examples 21-31, Comparative Examples 9-11.

In Examples 14, 21 and 22, and Comparative Examples 9 and 10, the amountof the cross-linkable monomer used was changed.

In Examples 23 to 26 and Comparative Example 11, the amount and type ofthe hydrophilic monomer were changed.

In Examples 27 to 29, the amount of toluene used was changed.

In Example 29, the shell of the hollow polymber particles was thin andthey were in an easily breakable condition.

In Example 30, geraniol as perfume was used as the oily substance. Inthis example, capsule-shaped polymer particles containing perfume wereobtained.

In Example 31, dibutyl phthalate was used as the oily substance. In thisexample, capsule-shaped particles containing dibutyl phthalate wereobtained.

                                      TABLE 3                                     __________________________________________________________________________                                        After Drying                                                Oily Substance                                                                          Particle Shape                                                                              Diameter                            Monomer Composition     Amount                                                                            after         Ratio                               (parts)           Type  (parts)                                                                           Polymerization                                                                        Particle                                                                            (μm/μm)                       __________________________________________________________________________    Ex. 14                                                                            MMA/ST/DVB = 80/10/10                                                                       toluene                                                                            20   A       B     0.51/0.3                            Ex. 21                                                                            MMA/ST/DVB = 80/19/1                                                                        toluene                                                                            20   A       B     0.50/0.3                            Ex. 22                                                                            MMA/ST/DVB = 25/35/50                                                                       toluene                                                                            20   A       B     0.50/0.3                            Com.                                                                              MMA/ST/DVB = 40/0/60                                                                        toluene                                                                            20   E       E     0.52/--                             Ex. 9                                                                         Com.                                                                              MMA/ST/DVB = 80/20/0                                                                        toluene                                                                            20   D       D     0.50/--                             Ex. 10                                                                        Ex. 23                                                                            MMA/ST/DVB = 40/50/10                                                                       toluene                                                                            20   A       B     0.48/0.3                            Ex. 24                                                                            MMA/ST/DVB = 20/70/10                                                                       toluene                                                                            20   A       B     0.52/0.3                                                                (slightly                                                                     deformed)                                 Com.                                                                              MMA/ST/DVB = 0/90/10                                                                        toluene                                                                            20   F       F     0.53/--                             Ex. 11                                                                        Ex. 25                                                                            MA/ST/DVB = 5/85/10                                                                         toluene                                                                            20   A       B     0.49/0.3                            Ex. 26                                                                            HEM/ST/DVB = 40/40/20                                                                       toluene                                                                            20   A       B     0.51/0.3                            Ex. 27                                                                            MMA/ST/DVB = 60/20/20                                                                       toluene                                                                            100  A       B     0.59/0.5                            Ex. 28                                                                            "             toluene                                                                            300  A       B     0.68/0.6                            Ex. 29                                                                            "             toluene                                                                            600  A       B     0.85/0.8                            Ex. 30                                                                            MMA/ST/DVB = 80/10/10                                                                       geraniol                                                                           20   A       A     0.50/0.3                            Ex. 31                                                                            "             dibutyl                                                                            20   A       A     0.50/0.3                                              phthalate                                                   __________________________________________________________________________     MMA: Methyl methacrylate                                                      ST: Styrene                                                                   DUB: Divinylbenzene                                                           MA: Methacrylic acid                                                          HEM: 2hydroxyethyl methacrylate                                               A, B, D, E, F: Same as defined in Table 1                                

EXAMPLES 32 TO 39

Polymer particles were produced in the same manner as in Example 14except that as seed polymer particles, 10 parts of the same particles asused in Example 14 were used, and as the monomer and the oily substance,those shown in Table 4 were used.

They are called Examples 32-39.

                                      TABLE 4                                     __________________________________________________________________________                                        After Drying                                                Oily Substance                                                                          Particle Shape                                                                             Diameter                             Monomer Composition    Amount                                                                             after        Ratio                                (parts)           Type (parts)                                                                            Polymerization                                                                        Particle                                                                           (μm/μm)                        __________________________________________________________________________    Ex. 32                                                                            MMA/ST/DVB = 80/10/10                                                                       benzene                                                                            20   A       B    0.50/0.3                             Ex. 33                                                                            "             p-xylene                                                                           20   A       B    0.48/0.3                             Ex. 34                                                                            "             cyclo-                                                                             20   A       B    0.53/0.3                                               hexane                                                      Ex. 35                                                                            AA/ST/TMP = 5/85/10                                                                         n-hexane                                                                           20   A       B    0.51/0.3                             Ex. 36                                                                            MMA/ST/EGD = 80/10/10                                                                       p-xylene                                                                           20   A       B    0.47/0.3                             Ex. 37                                                                            VP/ST/DVB = 40/50/10                                                                        toluene                                                                            20   A       B    0.52/0.3                             Ex. 38                                                                            DAE/ST/DVB = 80/10/10                                                                       toluene                                                                            20   A       B    0.53/0.3                             Ex. 39                                                                            GM/ST/DVB = 80/10/10                                                                        toluene                                                                            20   A       B    0.50/0.3                             __________________________________________________________________________     MMA: Methyl methacrylate                                                      ST: Styrene                                                                   DVB: Divinylbenzene                                                           AA: Acrylic acid                                                              TMP: Trimethylpropane trimethacrylate                                         EGD: Ethylene glycol dimethacrylate                                           VP: Vinylpyridine                                                             DAE: Dimethylaminoethyl methacrylate                                          GM: Glycidyl methacrylate                                                     A, B: Same as defined in Table 1                                         

EXAMPLES 40 AND 41, AND COMPARATIVE EXAMPLE 12

Polymer particles were produced in the same manner as in Example 14except that toluene as the oily substance was not used, thepolymerization time was adjusted to 3 hours, 4 hours or 5 hours, and thepolymer yield was controlled to 90%, 96% or 100%.

They are called Examples 40, 41 and Comparative Example 12.

These polymer particles were dried and examined with a transmission-typeelectron microscope. In Example 40, the polymer particles werecompletely spherical hollow particles having an outer diameter of 0.46μm and an inner diameter of 0.2 μm. In Example 41, the polymer particleswere completely spherical hollow particles having an outer diameter of0.48 μm and an inner diameter of 0.2 μm. However, in Comparative Example12, the polymer particles were solid particles having a particlediameter of 0.45 μm and no void was found in the inside of theparticles.

EXAMPLE 42

Twenty parts (calculated as solids) of an aqueous dispersion ofpolystyrene particles (particle diameter: 2 to 6 μm; average numbermolecular weight: 6,200) produced by suspension polymerization, 3 partsof polyvinyl alcohol and 0.5 part of sodium laurylsulfate were added to500 parts of water, and further a mixture of 45 parts of methylmethacrylate, 5 parts of divinylbenzene, 50 parts of styrene, 3 parts ofbenzoyl peroxide, and 30 parts of cyclohexane was added. The resultingmixture was stirred for 3 hours to make the monomer component and theoily substance almost absorbed in the polystyrene particles. Thetemperature of the system was raised to 80° C., and polymerization wasperformed for 6 hours. Then capsule-shaped polymer particles wereobtained in a polymer yield of 98%.

These capsule-shaped polymer particles were filtered off and then driedunder reduced pressure to obtain hollow polymer particles having aparticle diameter of 3 to 15 μm. With these polymer particles, the outerdiameter/inner diameter ratio was nearly 10/6.

EXAMPLE 43

Five parts of a styrene-butadiene rubber (SBR #1500 produced by JapanSynthetic Rubber Co., Ltd.) was dissolved in a mixture of 60 parts ofmethyl methacrylate, 35 parts of styrene, 5 parts of divinylbenzene and2 parts of benzoyl peroxide.

The solution thus prepared was added to 1,000 parts of water with 10parts of polyvinyl alcohol dissolved therein and stirred, and thenpolymerization was performed at 60° C. for 3 hours. When the polymeryield reached 83%, the reaction system was quickly cooled to 20° C. As aresult, a dispersion of capsule-shaped particles containing methylmethacrylate in the inside thereof and having a particle diameter of 5to 10 μm were obtained in a polymer yield of 86%.

These polymer particles were subjected to steam stripping treatment andthen dried to obtain hollow particles having an outer diameter/innerdiameter ratio=10/6.

EXAMPLE 44

As seed polymer particles, an emulsion of polymer particles having amonomer composition of styrene/methyl methacrylate/acrylic acid=20/78/2,a number average molecular weight of 3,800, and a particle diameter of0.16 μm was used. Five parts (calculated as solids) of the emulsion wasdispersed in 200 parts of an aqueous solution in which 0.7 part ofpolyoxyethylene nonylphenyl ether, 0.3 part of sodiumdodecylbenzenesulfonate, and 0.5 part of potassium persulfate had beendissolved.

As the monomer component, 20 parts of tetramethylolmethane triacrylate(NK Ester TMM-50T produced by Shin Nakamura Kagaku Kogyo Co., Ltd.;effective component: 50%; the remainder: toluene), 30 parts ofvinylpyridine, and 60 parts of styrene were added. As the oilysubstance, toluene as a diluent used in NK Ester was used, and anyadditional oily substance was not used. The mixture was stirred at 40°C. for 30 minutes to make the oily substance and the monomer componentabsorbed in the polymer particles. Then the temperature was raised to70° C., and polymerization was performed for 6 hours. Thus an emulsionof polymer particles was obtained in a polymer yield of 98%.

To this emulsion was added 20 parts of a 1% aqueous solution of calciumchloride to coaggrerate the particles. These particles were separated byfiltration. On removing the water and toluene from the particles bydrying in an infrared heating furnace maintained at 90° to 140° C.,hollow polymer particles having an outer diameter of 0.42 μm and aninner diameter of 0.15 μm were obtained.

EXAMPLE 45

To a mixture of 25 parts (10 parts as solids) of the same seed polymerparticle dispersion as used in PG,52 Example 14, 1 part of sodiumdodecylbenzenesulfonate, 2 parts of polyethylene glycol monostearate,and 100 parts of water were added the following compounds:

    ______________________________________                                        Methyl methacrylate   70       parts                                          Styrene               10       parts                                          Toluene               20       parts                                          Tert-butylperoxy 2-ethylhexanoate                                                                   2        parts                                          ______________________________________                                    

The resulting mixture was stirred to make the above compounds absorbedin the seed polymer particles.

Ten liters of the above mixture and 10 liters of water were placed in a120-liter polymerization reactor equipped with a stirrer and a coolingdevice. The temperature of the system was raised to 75° C. to initiatepolymerization. After one hour, polymerization was performed whileintroducing a fresh mixture at a rate of 10 liters per hour over 9hours. After 12 hours, polymerization was completed to yield adispersion of polymer particles. During the polymerization process, thetemperature in the polymerization reactor could be maintained at 75° C.in a stabilized manner. The polymer yield was 99%.

Then the dispersion was subjected to steam stripping treatment. Therewere obtained capsule-shaped polymer particles having an outer diameterof 0.51 μm and an inner diameter of 0.2 μm.

Industrial Applicability

In accordance with the present invention, capsule-shaped polymerparticles in the inside of which phase-separation between a polymerphase constituting the shell and an oily substance phase constitutingthe core is surely carried out, and a process for the production ofcapsule-shaped polymer particles whereby the above capsule-shapedpolymer particles can be easily produced by a simplified process can beprovided.

Furthermore, in the present invention, by using polymer particles havinga small particle diameter as the different polymer to be incorporated inthe dispersion during polymerization, small-sized capsule-shaped polymerparticles which have heretofore been difficult to produce can be easilyproduced.

In the process of the present invention, capsulation is accomplishedsatisfactorily. Moreover, capsule-shaped polymer particles produced bythe process of the present invention are excellent in such properties asmechanical strength and heat resistance. These capsule-shaped polymerparticles can contain various substances such as solvents, plasticizers,perfume, ink and oil-soluble medicines in the inside thereof as the coreand, therefore, can be utilized in various fields.

Furthermore the present invention can provide water containing hollowpolymer particles or hollow polymer particles by removing the oilysubstance contained in capsule-shaped polymer particles produced by theabove process of production, and a process of production whereby thewater containing hollow polymer particles or hollow polymer particlescan be surely produced by a simplified process.

These hollow polymer particles possess unique optical characteristics,and high hiding power, high whiteness and excellent luster. Thus theycan be used as fillers of light weight, high absorbing capacity and highoil-absorbing capacity in various fields such as agents to be compoundedto paints and paper-coating compositions, water-absorbing fillers forink jet paper, and internal fillers for paper-making.

In the above Examples and Comparative Examples, all parts and % are byweight.

We claim:
 1. A process for producing capsule-shaped polymer particles,comprising the steps of:dispersing finely-divided particles of a polymerin water; adding a polymerizable monomer component comprising (i) across-linkable monomer and a hydrophilic monomer, and (ii) an oilsubstance to the water dispersion, whereby the monomer component and theoil substance are absorbed in the finely-divided polymer particles andsaid polymer particles are soluble in said polymerizable monomercomponent; and polymerizing said monomer component to producecapsule-shaped polymer particles containing said oily substance on theinside thereof, wherein said polymer is different than saidcapsule-shaped polymer resulting from said polymerizing step.
 2. Theprocess of claim 1, further comprisingremoving said oily substance toproduce capsule-shaped hollow or water containing polymer particles. 3.The process of claim 1, wherein said monomer component comprises 1-50 wt% of said cross-linkable monomer, 3-99 wt % of said hydrophilic monomer,and 0-85 wt % of a monomer copolymerizable with said cross-linkablemonomer or hydrophilic monomer, and wherein the amount of said polymeris 1-100 parts by weight per 100 parts by weight of the total wieght ofsaid monomer component.
 4. The process of claim 1, furthercomprisingintroducing a substance having a solubility in water of notmore than 10⁻³ wt % to the finely divided polymer particles prior tosaid adding step, whereby said substance is absorbed in said particles.5. The process of claim 1, wherein the number average molecular weightof said polymer is not more than 20,000.
 6. The process of claim 5,wherein the number average molecular weight of said polymer is not morethan 7,000.
 7. The process of claim 1, wherein said polymer has anaverage particle diameter of 0.1-0.4 μm.
 8. The process of claim 1,wherein said oily substance is the same as said cross-linkable monomeror hydrophilic monomer, whereby said monomer component is incompletelypolymerized so that a portion of said monomer component remainsunpolymerized on the inside of said polymer particle.
 9. The process ofclaim 1, wherein said cross-linkable monomer is at least one monomerselected from the group consisting of divinylbenzene, ethylene glycoldimethacrylate, and trimethylolpropane trimethacrylate; and saidhydrophilic monomer is at least one monomer selected from the groupconsisting of methyl methacrylate, vinylpyridine, 2-hydroxyethylmethacrylate, methacrylic acid, acrylic acid, glycidyl methacrylate, anddimethylaminoethyl methacrylate.
 10. The process of claim 1, whereinsaid crosslinkable monomer is divinylbenzene; said hydrophilic monomeris at least one monomer selected from the group consisting of methylmethyacrylate, 2-hydroxyethyl methacrylate, and methacrylic acid; andsaid polymer is at least one polymer selected from the group consistingof polystyrene, carboxy-modified polystyrene, styrene-butadienecopolymers, styrene-acrylate copolymers, styrene-methacrylatecopolymers, acrylate copolymers, methacrylate copolymers,carboxy-modified styrene-acrylate copolymers, carboxy-modifiedstyrene-methacrylate copolymers, carboxy-modified acrylate copolymers,and carboxy-modified methacrylate copolymers.
 11. The process of claim1, wherein said oily substance is at least one substance selected fromthe group consisting of toluene, benzene, xylene, cyclohexane, andn-hexane.
 12. The process of claim 2, wherein said removing step isperformed by steam stripping, gas blowing, or pressure reduction. 13.The process of claim 2, wherein said removing step is performed bydrying said capsule-shaped polymer particles after polymerization. 14.The process of claim 1, wherein said adding step is performedcontinuously or batchwise.